1. Field of the Invention
This invention relates generally to therapeutic placement of interventional medical devices into the vasculature of the human body. More particularly, this invention concerns a placement system using heat-activated shape memory materials to release endoluminal devices at a desired location within the body.
2. Description of Related Art
A type of interventional medical device known as an endoluminal coil is used for a wide variety of therapeutic purposes including the treatment of intracranial vascular aneurysms. A vascular aneurysm is often formed as the result of an abnormal dilation of a blood vessel which weakens the arterial wall and allows it to expand into an adjacent body tissue or cavity. Intracranial aneurysms may be treated to prevent rupturing by placing endoluminal coils through the neck of an opening from the vessel into the interior cavity of the aneurysm. After placement, the coils pose a physical barrier, reducing blood flow into the aneurysm and promoting the formation of an embolus in the aneurysm cavity. The embolus formation in the aneurysm cavity further impedes blood flow into the aneurysm and reduces the blood pressure against the aneurysm wall, thus reducing the possibility of a rupture.
One known method for delivering coils into an intracranial aneurysm involves the use of a catheter and a guidewire with a detachable tip shaped in the form of a coil. Such a system is described in U.S. Pat. No. 5,312,415 which is incorporated herein by reference. Microcatheters are known, for example, that allow for navigation into cerebral arteries and entry into intracranial aneurysms. The catheter is guided through the vasculature using a guidewire until it reaches the desired location. The tip of the guidewire is then detached and the coils are pushed into the aneurysm until they occlude at least a portion of the interior of the aneurysm. Although generally effective, this technique has limitations on the accuracy for precision placement of embolic coils in intracranial aneurysms. It would be particularly desirable to have a simple delivery system which allows for precise positioning of embolic coils and virtually instantaneous release once the coils are in place.
One approach which provides for greater accuracy of placement involves attaching a coil to the end of a guidewire and maneuvering the guidewire to place the embolic coil in the desired location and then releasing the coil from the guidewire. Since the physician has control of the guidewire and the coil is firmly attached to the distal end of the pusher, it is possible to achieve a much higher degree of placement accuracy with this approach. However, to implement this approach, the delivery system must include a release mechanism which can be used to easily decouple the coil from the guidewire while inside tiny blood vessels. A variety of release mechanisms have been proposed for this purpose.
In one known technique for deploying an endoluminal device, endoluminal coils may be released through electrolytic dissolution of a connecting segment between the coil and the distal tip of the guidewire. This method typically involves the application of a positive direct current for a predetermined amount of time which results in the dissolution of a stainless steel connector which holds the coil to the guidewire. Although this method has met with considerable success, the procedure has significant disadvantages. Because the use of electrolytic dissolution is slow and unreliable, the delivery of the devices can be very time consuming and therefore very costly. The increased surgery time also creates a higher risk for the patient. In addition, the secondary effects of dissolving a stainless steel wire in the blood could possibly be detrimental to the patient. For these reasons, a simpler, faster, safer and more reliable method of delivering the devices is needed.
Detachable coil assemblies are also known that use a threaded coupling such that the coil is released when the guidewire is rotated. Another conventional technique uses a heat-releaseable adhesive bond to separate the coils from the distal end of the catheter. When laser energy is transferred via a fiber optic cable to the connector, the connector is heated, thereby releasing the heat-sensitive adhesive bond between the connector and coil.
One known implant delivery assembly is activated thermally, and includes a coupling portion made of a shape memory material that interlockingly engages the implant when the shape memory material is in one configuration, and releases the implant in another configuration. The implant is detachably coupled to a pusher formed of shape memory material that allows thermal activation of the decoupling mechanism. The coupling portion is constructed with a deformed shape for holding the implant to the pusher, and a pre-set shape that provides release of the implant when the thermal activation is provided. The coupling portion of the pusher is heated by passing an electric current between the pusher and the body of the patient.
Another detachable embolic coil assembly is known that uses interlocking clasps that are used in a surgical instrument for delivering an embolic coil to a selected site within the vasculature of the human body.
Yet another known embolic coil assembly includes a ball that is forced through an aperture in a socket on the distal end of a pusher to release the coil. After a catheter is inserted and navigated through the vessel, and the coil is in place, a plunger is advanced to press the ball and its coil into the target site.
Some conventional vasoocclusive devices are operated by pulling or jerking the catheter tip from an inflatable balloon, thus potentially compromising the position of the implant. One such device provides for an endovascular wire and tip that can be separated from the holding wire mechanically or electrolytically for the formation of thrombus in blood vessels. However, such devices that release the interventional device by mechanically breaking an intermediate section between the catheter tip and balloon can potentially leave broken or jagged ends that could injure the vasculature.
One conventional releaseable balloon catheter used to embolize vascular lesions has a tubular portion made of a material such as a hydrophilic polymer located between the catheter and the balloon that can be broken by torsion of the tubular portion. The tubular portion can be melted by heating the tubular portion, or can be dissolved in the blood when heated, and electrodes are provided for heating the tubular portion. Another conventional technique for separating a balloon from a balloon catheter involves the melting and breaking of a connecting member made from polyvinyl alcohol or trans-polyisoprene between the balloon and the catheter body when power is supplied to electrodes provided for heating the connecting member. When the connecting member is heated to temperatures of about 70.degree. C. and slight tension is applied, the balloon can be separated from the main catheter body. However, such devices that release the interventional device by melting or dissolving the intermediate section between the catheter tip and balloon can also potentially release undesirable particles from the connecting member into the bloodstream.
From the above, it can be seen that a variety of approaches to placing embolic devices have been developed, but all of them are limited in some way by the time to release, the dispersion of particles or chemicals, the introduction of electricity, mechanical force on the implant after placement, or some combination of these affects. There is therefore a need for a precise, controlled method of deploying therapeutic interventional devices without compromising the position of the implant, without presenting broken or jagged ends that can potentially injure the vasculature, and without release undesirable particles or materials into the bloodstream.
Recently, a release system for vasoocclusive coils has been developed involving the use of a microgripper made of shape memory material. The shape memory microgrippers is mechanically actuated by the conversion of laser light to heat energy. Another newly developed type of release mechanism using shape memory materials involves a tube of radially recovering shape memory polymer attached to the distal end of an optical fiber pusher. A device such as an endoluminal coil is introduced into the tube and the tube is compressed or crimped around the end of the coil to hold it in place. Once the coil is in the desired location in the vasculature, the tube of shape memory polymer is heated by passing light through the optical fiber pusher to the distal end of the pusher, thereby causing the tube to recover its original diameter and shape. After the tube has recovered its original shape, it is no longer compressed or crimped around the device and the device is free to slip out of the tube.
In another approach, an endoluminal coil delivery system is provided with a mechanical release mechanism for positioning and delivering a coil within a lumen that utilizes a mechanical latch to engage the coil during positioning. The coil is placed at the distal end of delivery system and includes a fitting at the end of coil which is engaged by jaws. The coil is released from the jaws by advancing a release tube over the jaws, which squeezes the jaws, thereby disengaging them from the fitting.
A shape memory metal actuated separation device is also known that can be used for spacecraft. A segmented nut engages a threaded bolt that is to be held and released and is held together by a nut retainer that is movable with respect to the nut and affixed to a shape memory alloy element. The shape memory alloy element is heated by an electrical resistance heater, thereby moving the retainer which causes disengagement.
In one coil shaped intravascular stent formed into a coil spring, to be used to reinforce an arterial wall, the wire forming the stent has axially spaced rollers or bearings to facilitate advancement and withdrawal of the coil spring, with enlarged beads between the rollers to hold the rollers away from one another.
While the use of shape memory polymers for release of vasoocclusive coils has shown great promise, the use of radially recovering shape memory polymers has not been completely satisfactory; the primary problem recently encountered with radially recovering tubes being that shape memory polymers tend to remold themselves around devices which exert external forces upon them, the tube of shape memory polymer then tending to remold itself around the endoluminal coil thereby causing problems during the attempted release. In a situation in which the device does not completely detach from the pusher, the coil may be inadvertently pulled part or all of the way back out of the vasculature malformation and into the parent vessel or the vasculature. Since proper positioning of the intravascular device is critical to the successful treatment of intracranial aneurysms, a reliable method of delivering and placing the devices is critically important. The present invention meets these and other needs.